This invention is concerned with the detection of electromagnetic radiation, and in particular with detectors capable of detecting individual photons.
The detection of electromagnetic energy is made possible because of a series of complex interactions which occur between such energy and certain kinds of materials. These interactions enable remote sensing systems to record contrasts between an observed object and its background. In using radiation for detection, however, the dual nature of electromagnetic energy must be recognized. Electromagnetic radiation characteristically exhibits both wave and particle like behavior, with the photon model (which represents the discrete amount of energy associated with radiation at a particular wavelength) emphasizing the quantized and statistical properties of electromagnetic radiation, while a wave description of such radiation stresses overall, average effects.
One limit to the detection of electromagnetic energy is established by the uncertainty principle of quantum mechanics, which prohibits the simultaneous measurement of both the number of photons (intensity) and the phase of an electromagnetic field. A direct (incoherent) detector, however, does not measure phase, but produces an output voltage or current which is proportional to the photon flux. Hence the uncertainty principle places no restriction on the measurement of photons by a direct detector and such a detector can, in principle, be completely noiseless and detect the arrival of a single photon.
In a situation where a large amount of background radiation is present, the need for such an ideal photon counter is not acute--a detector with relatively low responsivity but unit quantum efficiency can perform as well as an ideal detector under background-limited conditions. In a low background environment, however, the fluctuations in the background radiation are so small that the responsivity of the detector becomes more important than quantum efficiency. Here the ability to count individual photons would be highly desirable. Many important applications, in fields such as astronomy and spectroscopy, involve the regime of low-background detection and would find many uses for a detector capable of counting individual photons, particularly in the infrared and longer wavelength regions of the electromagnetic spectrum. The adoption of such a detector in the space environment, for example, would create an opportunity for dramatic improvements in astronomical observations in the 1-1000 um wavelength range. See, e.g., Richards, et al., Infrared Detectors for Low-Background Astronomy: Incoherent and Coherent Devices from One Micrometer to One Millimeter, in Infrared and Millimeter Waves-Systems and Components, Vol. 6, Page 149 (K. Button ed. 1982). Unfortunately, however, the devices which have traditionally been available in the art for the detection of incoherent infrared radiation are at least two to three orders of magnitude short of the ability to count single photons. Some of the limitations of such prior art devices will be apparent from a brief review of the field.
In general, electromagnetic radiation can interact with materials by virtue of photon effects, thermal effects, or wave interactions. Of these categories, photon effects are most important in the realm of photon detection. The class of photon effects includes all interactions between incident photons and electrons within a material, whether the electrons are bound to lattice atoms or free. Photon effects may be further categorized as either internal or external. In the external or photoemissive effect, the incident photon causes an emission of an electron from the surface of the absorbing material (the photocathode). Photomultipliers utilizing this effect have been employed in astronomical detectors at wavelengths less than 1 um, where each free photoelectron may be accelerated to a high energy by an electric field and detected as a single event. These devices have not been successfully applied, however, to wavelengths significantly beyond 1 um.
An internal photon effect involves a photoexcited carrier (an electron or hole) which remains within the material. The most common types of internal effects are photoconductivity and photovoltaic effects, with photoconductivity being the most widely used. Intrinsic photoconductivity requires the excitation of a free hole-electron pair by a photon with an energy at least as great as the energy gap of the host material, while extrinsic photoconductivity involves the absorption of an incident photon at a neutral impurity center, thereby forming either a free electron or a free hole and leaving the impurity center in an ionized state. High performance intrinsic detectors suitable for low background conditions are generally available only for the wavelength region below 15-20 um, while extrinsic detectors in Si and Ge can operate at wavelengths up to 210 um. Photoconductive gains greater than one require materials with long lifetimes and devices equipped with ohmic contacts, which allow the free passage of carriers from one electrode into the semiconductor to replenish those carriers removed at the other electrode. The minimum photon rate which has been detected with internal detectors, however, is approximately 1000 photons/sec-Hz.sup.1/2 with a 1 Hz bandwidth.
Photovoltaic devices comprise another common type of internal detector. The photovoltaic mechanism requires an internal potential barrier with a built-in electric field to separate a photoexcited hole-electron pair. One example of such a device is the avalanche photodiode (APD), which includes an internal gain mechanism, making it somewhat analogous to a photomultiplier but with the potential for a higher quantum efficiency and a larger bandwidth. The APD utilizes avalanche breakdown, which occurs in a p-n junction of moderate doping levels under reverse bias. In the absence of radiation, the thermally excited carriers normally present in the semiconductor are accelerated within the high field region of the junction to velocities so high that their collisions with lattice atoms transfer electrons by impact ionization from the valence to the conduction band, leaving free holes in the valence band. These freed electrons are then accelerated, collide with other atoms, and free more electron-hole pairs. Thus an avalanche of electrons occurs within the high field region of the junction.
Because avalanching can be initiated by photoexcited electrons or holes, as well as by thermally excited ones, the effect produces an increase in the number of photoexcited carriers. An avalanche photodetector, however, cannot be supplied with sufficient gain to detect single photons. This gain (the number of electrons flowing through the detector circuit per carrier generating photon) must be greater than 10.sup.4. (See A. Rose, An Analysis of Photoconductive Photon Counting, Proc. 3rd Photoconductivity Conf., Page 7 (E. Pell ed. 1969)). In addition, because the impact ionization effect in such a device occurs across the bandgap, the impacting electrons generate both electrons and holes, which each can cause additional electron-hole pairs by further impact ionization. As a result, an avalanche device is basically unstable and statistical variations in the impact ionization process can cause large fluctuations in the gain or multiplication of the device, contributing considerable excess noise.
Because of limitations such as those discussed above, the detectors available in the art are not capable of detecting individual photons in a wide variety of applications where such a detecting capability would be very useful and would facilitate the accomplishment of tasks which cannot be achieved with currently available devices. Such a detection technique would be well received, for example, in the field of infrared astronomy and would find immediate acceptance for use in spectroscopic techniques such as grating spectroscopy, Fourier transform spectroscopy, and Fabry-Perot interferometry.